32‐Channel self‐grounded bow‐tie transceiver array for cardiac MR at 7.0T

Abstract

Purpose Design, implementation, evaluation, and application of a 32‐channel Self‐Grounded Bow‐Tie (SGBT) transceiver array for cardiac MR (CMR) at 7.0T. Methods The array consists of 32 compact SGBT building blocks. Transmission field (B1+) shimming and radiofrequency safety assessment were performed with numerical simulations and benchmarked against phantom experiments. In vivo B1+ efficiency mapping was conducted with actual flip angle imaging. The array’s applicability for accelerated high spatial resolution 2D FLASH CINE imaging of the heart was examined in a volunteer study (n = 7). Results B1+ shimming provided a uniform field distribution suitable for female and male subjects. Phantom studies demonstrated an excellent agreement between simulated and measured B1+ efficiency maps (7% mean difference). The SGBT array afforded a spatial resolution of (0.8 × 0.8 × 2.5) mm3 for 2D CINE FLASH which is by a factor of 12 superior to standardized cardiovascular MR (CMR) protocols. The density of the SGBT array supports 1D acceleration of up to R = 4 (mean signal‐to‐noise ratio (whole heart) ≥ 16.7, mean contrast‐to‐noise ratio ≥ 13.5) without impairing image quality significantly. Conclusion The compact SGBT building block facilitates a modular high‐density array that supports accelerated and high spatial resolution CMR at 7.0T. The array provides a technological basis for future clinical assessment of parallel transmission techniques.

Scaling the mountains: what lies above 7 Tesla magnetic resonance?

“…To harness the potential of higher field MRI systems, substantial efforts have been dedicated to developing radiofrequency (RF) coil technology and related methodology that can cope with the challenges of RF transmission field inhomogeneity and increased RF power deposition. A large body of work has already provided enormous progress and results that robustly demonstrate optimal and safe human MR scanning at 7 T, 9.4 T and 10.5 T [8,[23][24][25][26][27]. This includes implementation of multi-channel transmit-receive RF arrays, approaching the ultimate signal-to-noise ratio, improvements in RF transmission field distribution and enhanced SAR prediction and management [28].…”

“…This makes RF technology established for 1 H MR at 7 T very well suited to be adapted and fine-tuned for hetero-nuclear MR at 14 T or 20 T. Leveraging this advantage, MR at 14 T or 20 T is conceptually appealing to improve our understanding of ion homeostasis and energy metabolism in vivo in humans, as outlined in [43]. For example, the sensitivity gain at 20 T is expected to reduce scan times for 31 P and 23 Na by a factor of 8-10 versus approaches outlined in this Special Issue for 7 T [44]. Implications of these gains in sensitivity and speed include the promise of sodium MR of the heart with a sub-millimetre spatial resolution in 5-10 min scan time, and the potential for probing cardiac metabolism with 31 P MR spectroscopy in clinically acceptable examination times.…”

Ultra-high field cardiac MRI in large animals and humans for translational cardiovascular research

“…In the future it will allow the use of -shimming and parallel transmission (pTx) methodology. The latter two techniques are considered necessary requirements for good cardiac image quality although methodologies need to be developed further ( 25 , 27 , 53 ).…”

“…Cardiac 7 T MRI in humans, however, is still not widely available because of methodological and technological challenges at high fields which are still an active area of research: (i) B 0 ( 22 ) and (i.e., of the high-frequency transmit field) inhomogeneities, (ii) excessive tissue heating, (iii) limited availability of optimized commercial pulse sequences ( 23 ), (iv) limited availability of commercial RF coil concepts for body ( 24 ) and cardiac ( 25 , 26 ) imaging, (v) parallel transmit technology ( 27 ), (vi) unclear and not well-established safety concepts ( 28 ), (vii) and detrimental effects of the high field strength on the electrocardiogram ( 29 – 33 ) which may result in suboptimal synchronization of imaging with cardiac motion.…”

In Vivo Magnetic Resonance Spectroscopy Methods for Investigating Cardiac Metabolism

“…These studies aim to eventually include dynamic shim updating on the selected volume in the heart, as has been experimented with within the brain studies at ultra-high-field ≥7 Tesla [67,68]. Overall considerations on the heart shimming include: (i) using slab-selective approaches shimming to minimize the field distribution variations (shimming standard deviation (SD)), (ii) optimizing high-order shims, (iii) temporal B 0 -field variations with regards to cardiac cycle and respirationinduced, (iv) shimming of oblique orientations, (v) develop dynamic shimming interface by rapid field mapping using slab-specific or cardiac phase-specific mapping sets, and (vi) the emerging use of artificial intelligence in compensating the field variations [45,63,66,[69][70][71].…”